U.S. patent number 6,292,701 [Application Number 09/362,891] was granted by the patent office on 2001-09-18 for bipolar electrical stimulus probe with planar electrodes.
This patent grant is currently assigned to Medtronic Xomed, Inc.. Invention is credited to David C. Hacker, Richard L. Prass.
United States Patent |
6,292,701 |
Prass , et al. |
September 18, 2001 |
Bipolar electrical stimulus probe with planar electrodes
Abstract
A hand-held bipolar electrical stimulus probe includes first and
second insulated conductors having unequal diameters and a stable
side-by-side orientation. The conductors are provided with visually
perceptible indicia such as color coding for identification. The
conductors are carried within a flexible jacket and the terminal
portions thereof are deformable. The conductors are each insulated
substantially up to their tips to prevent current shunting. The
probe may include a momentary contact switch permitting selective
monopolar or bipolar operation.
Inventors: |
Prass; Richard L. (Virginia
Beach, VA), Hacker; David C. (Jacksonville, FL) |
Assignee: |
Medtronic Xomed, Inc.
(Jacksonville, FL)
|
Family
ID: |
26791486 |
Appl.
No.: |
09/362,891 |
Filed: |
July 29, 1999 |
Current U.S.
Class: |
607/116; 607/145;
607/150 |
Current CPC
Class: |
A61N
1/0551 (20130101) |
Current International
Class: |
A61N
1/05 (20060101); A61N 001/04 (); A61N 001/05 () |
Field of
Search: |
;607/115,116,145,149,150 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Layno; Carl
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application claims benefit from Provisional Application No.
60/096,243, filed Aug. 12, 1998, the entire disclosure of which is
incorporated herein by reference.
Claims
What is claimed is:
1. A hand-held instrument for monitoring or electrically
stimulating exposed, subcutaneous tissue of a living body,
comprising:
an elongate cannula having an open proximal end, an open distal end
and a lumen there between;
a handle carried on said cannula proximal end;
an elongate cathode conductor including a proximal end and a distal
end having a distal end surface having a first diameter, said
cathode conductor having sufficient rigidity to provide resistance
to bending when in operable contact with subcutaneous tissue;
said cathode conductor being disposed within a first flexible
insulating sheath;
an elongate anode conductor including a proximal end and a distal
end having a planar distal end surface having a second diameter
larger than said cathode conductor distal end first diameter, said
anode conductor having sufficient rigidity to provide resilient
resistance to bending when in operable contact with subcutaneous
tissue;
said anode conductor being disposed within a second flexible
insulating sheath;
said cathode conductor and said anode conductor being carried
within said cannula lumen, wherein said cathode conductor distal
end and said anode conductor distal end project distally beyond
said cannula open distal end.
2. The hand-held instrument of claim 1, said first flexible
insulating sheath including a colored surface.
3. The hand-held instrument of claim 2, said first flexible
insulating sheath including a black surface.
4. The hand-held instrument of claim 1, said cathode conductor and
said anode conductor being carried within an elongate
non-conductive, flexible jacket having a distal end, wherein said
cathode conductor distal end and said anode conductor distal end
project distally beyond said jacket distal end; and
said flexible jacket, cathode conductor and said anode conductor
being carried within said cannula lumen, wherein said flexible
jacket distal end, said cathode conductor distal end and said anode
conductor distal end project distally beyond said cannula open
distal end.
5. The hand-held instrument of claim 4, said flexible segment
extending beyond said cannula distal end in an unsupported
malleable length; said jacket unsupported malleable length being in
the range of one half inch to one inch.
6. The hand-held instrument of claim 1, wherein said cathode and
anode distal end surfaces lie in a plane disposed at an angle to
the elongate cathode conductor axis.
7. The hand-held instrument of claim 6, wherein said plane is
disposed at an angle fifteen degrees from normal to the elongate
cathode conductor axis.
8. A hand-held instrument for monitoring or electrically
stimulating exposed, subcutaneous tissue of a living body,
comprising:
an elongate cannula having an open proximal end, an open distal end
and a lumen there between;
a handle carried on said cannula proximal end;
an elongate, flexible, cathode conductor including a proximal end
and a distal end having a substantially planar distal end surface,
said cathode conductor being malleably deformable and having
sufficient resilience to resist bending when in operable contact
with subcutaneous tissue;
said cathode conductor being disposed within a first flexible
insulating sheath extending distally to and substantially flush
with the distal end of said cathode conductor, said sheath leaving
said cathode conductor planar distal end surface exposed;
an elongate, flexible anode conductor including a proximal end and
a distal end having a planar distal end surface, said anode
conductor being malleably deformable and having sufficient
resilience to resist bending when in operable contact with
subcutaneous tissue;
said anode conductor being disposed within a second flexible
insulating sheath extending distally to and substantially flush
with the distal end of said anode conductor, said sheath leaving
said anode conductor planar distal end surface exposed;
wherein at least one of said cathode conductor and said anode
conductor include visible identifying indicia; and
said cathode conductor and said anode conductor being carried
within said cannula lumen, wherein said cathode conductor distal
end and said anode conductor distal end project distally beyond
said cannula open distal end.
9. The hand-held instrument of claim 8, said visible identifying
indicia comprising a color coded flexible insulating sheath.
10. The hand-held instrument of claim 9, said color coded flexible
insulating sheath comprising a black plastic insulating sheath
disposed on said cathode conductor.
11. The hand-held instrument of claim 10, said handle further
including a tactile salient feature aligned with said cathode
conductor.
12. A hand-held instrument for monitoring or electrically
stimulating exposed, subcutaneous tissue of a living body,
comprising:
a handle having a proximal end and a distal end;
an elongate, flexible, cathode conductor including a proximal end
and a distal end having a distal end surface, said cathode
conductor having sufficient resilience to resist bending when in
operable contact with subcutaneous tissue;
said cathode conductor being disposed within an insulating sheath,
said sheath leaving said cathode conductor distal end surface
exposed;
an elongate, flexible anode conductor including a proximal end and
a distal end having a distal end surface, said anode conductor
having sufficient resilience to resist bending when in operable
contact with subcutaneous tissue;
said anode conductor being disposed within an insulating sheath,
said sheath leaving said anode conductor distal end surface
exposed;
said handle further including a switch having first and second
terminals; said first switch terminal being connected to one of
said cathode conductor and said anode conductor; and
said cathode conductor and said anode conductor being carried by
said handle, wherein said cathode conductor distal end and said
anode conductor distal end project distally beyond said handle
distal end.
13. The hand-held instrument of claim 12, said first switch
terminal being connected to said anode conductor.
14. The hand-held instrument of claim 13, further including a
remote anode electrode;
said second switch terminal being connected to said remote anode
electrode.
15. The hand-held instrument of claim 14, said switch being
actuable by a pushbutton carried externally on said handle.
16. The hand-held instrument of claim 12, said switch being a
momentary contact, single-pole single-throw switch.
17. The hand-held instrument of claim 10, wherein said instrument
operates in a bipolar mode when said switch is actuated.
18. An instrument for monitoring or electrically stimulating
exposed, subcutaneous tissue of a living body, comprising:
an elongate cathode conductor including a proximal end and a distal
end having a distal end surface, said cathode conductor having
sufficient resilience to resist bending when in operable contact
with subcutaneous tissue, said cathode conductor having a
diameter;
said cathode conductor being disposed within a first insulating
sheath, said first sheath extending substantially to the cathode
conductor distal end and leaving said cathode conductor distal end
surface exposed;
said cathode conductor distal end surface defining a substantially
planar distally exposed cathode tip;
an elongate anode conductor including a proximal end and a distal
end having a distal end surface, said anode conductor having
sufficient resilience to resist bending when in operable contact
with subcutaneous tissue, said anode conductor having a diameter
different from said diameter of said cathode conductor;
said anode conductor being disposed within a second insulating
sheath, said second sheath extending substantially to the anode
conductor distal end and leaving said anode conductor distal end
surface exposed; and
said anode conductor distal end surface defining a substantially
planar distally exposed anode tip, wherein said substantially
planar distally exposed anode tip and said substantially planar
distally exposed cathode tip are in a substantially coplanar flush
distal tip configuration.
19. The instrument of claim 18, said cathode conductor including a
selectively bendable malleable terminal portion proximate said
distal end, said malleable portion adapted to resiliently retain a
selected bent shape;
said anode conductor including a selectively bendable malleable
terminal portion proximate said distal end, said malleable portion
adapted to resiliently retain a selected bent shape; and
said cathode and anode conductors being disposed in a flexible
jacket, said jacket extending over a selected fraction of the
length of the cathode and anode malleable terminal portion and
maintaining a selected spacing between said flush distal tips of
said anode and cathode conductors.
20. The instrument of claim 18, said cathode conductor having a
first diameter and said anode conductor having a second diameter
larger than said first diameter.
21. An instrument for monitoring or electrically stimulating
exposed, subcutaneous tissue of a living body, comprising:
an elongate cathode conductor including a wire having a proximal
end and a distal end having a distal end surface, said wire of said
cathode conductor having sufficient resilience to resist bending
when in operable contact with subcutaneous tissue;
said wire of said cathode conductor being malleable along a
selectively bendable malleable terminal portion proximate said
distal end, said malleable terminal portion being adapted to
resiliently retain a selected bent shape;
an elongate anode conductor including a wire having a proximal end
and a distal end having a distal end surface, said wire of said
anode conductor having sufficient resilience to resist bending when
in operable contact with subcutaneous tissue;
said wire of said anode conductor being malleable along a
selectively bendable malleable terminal portion proximate said
distal end of said wire of said anode conductor, said malleable
terminal portion of said anode conductor being adapted to
resiliently retain a selected bent shape; and
said wires being sheathed in a flexible jacket, said jacket
extending over a selected fraction of the length of said malleable
terminal portions and maintaining a selected side by side spacing
between said distal end surfaces of said wires.
22. The instrument of claim 21, said flexible jacket extending over
four-fifths of said malleable terminal portions.
23. The instrument of claim 21, said wire of said cathode conductor
having a first diameter and said wire of said anode conductor
having a second diameter larger than said first diameter.
24. The instrument of claim 21, said wire of said cathode conductor
carrying an insulating sheath having a first color and said wire of
said anode conductor carrying an insulating sheath having a second
color visually distinguishable from said first color.
25. The instrument of claim 21, said cathode conductor distal end
surface defining a substantially planar exposed cathode tip and
said anode conductor distal end surface defining a substantially
planar exposed anode tip in a substantially coplanar, flush distal
tip configuration with said cathode tip.
26. An instrument for monitoring or electrically stimulating
exposed, subcutaneous tissue of a living body, comprising:
an elongate cathode conductor including a proximal end and a distal
end having a distal end surface, said cathode conductor having
sufficient resilience to resist bending when in operable contact
with subcutaneous tissue;
an elongate anode conductor including a proximal end and a distal
end having a distal end surface, said anode conductor having
sufficient resilience to resist bending when in operable contact
with subcutaneous tissue;
said cathode conductor having a first diameter and said anode
conductor having a second diameter different than said cathode
first diameter.
27. The instrument of claim 26, said anode conductor second
diameter being larger than said cathode first diameter.
28. The instrument of claim 27, said anode conductor second
diameter being approximately twice said cathode conductor first
diameter.
29. The instrument of claim 27, said anode conductor second
diameter being 24 AWG and said cathode conductor first diameter
being 30 AWG.
30. The instrument of claim 26, said cathode conductor carrying an
insulating sheath having a first color and said anode conductor
carrying an insulating sheath having a second color visually
distinguishable from said cathode insulating sheath first
color.
31. The instrument of claim 26, said cathode conductor distal end
surface defining a substantially planar exposed cathode tip and
said anode conductor distal end surface defining a substantially
planar exposed anode tip in a substantially coplanar, flush distal
tip configuration with said cathode tip.
32. The instrument of claim 26, further including a bipolar
electrical cord having first and second flexible conductors, said
first flexible conductor having a first diameter and said second
flexible conductor having a second diameter different than said
first flexible conductor first diameter.
33. A hand-held instrument for monitoring or electrically
stimulating exposed, subcutaneous tissue of a living body,
comprising:
an elongate cannula having an open proximal end, an open distal end
and a lumen there between;
a handle carried on said proximal end;
a flexible, non-conductive, elongate jacket projecting distally
from said cannula distal end to terminate at a jacket distal
end;
an elongate, flexible cathode conductor disposed within said jacket
and extending distally from said cannula distal end to terminate at
a cathode conductor distal end disposed distally of said jacket
distal end, said cathode conductor distal end having a planar
distal end surface, said cathode conductor being malleably
deformable and having sufficient resilience to resist bending when
in operable contact with subcutaneous tissue;
said cathode conductor being disposed within a first flexible
insulating sheath extending distally from said jacket distal end to
terminate substantially flush with said distal end of said cathode
conductor, said first sheath leaving said cathode conductor planar
distal end surface exposed;
an elongate, flexible anode conductor disposed within said jacket
and extending distally from said cannula distal end to terminate at
an anode conductor distal end disposed distally of said jacket
distal end, said anode conductor distal end having a planar distal
end surface, said anode conductor being malleably deformable and
having sufficient resilience to resist bending when in operable
contact with subcutaneous tissue; and
said anode conductor being disposed within a second flexible
insulating sheath extending distally from said jacket distal end to
terminate substantially flush with said distal end of said anode
conductor, said second sheath leaving said anode conductor planar
distal end surface exposed.
34. The hand-held instrument of claim 33, said anode conductor and
said cathode conductor in said flexible jacket defining a malleable
distal probe segment; said malleable distal probe segment being
plastically deformable to a selected angular position, wherein said
malleable distal probe segment will, after deformation, resiliently
maintain said selected angular position.
35. The hand-held instrument of claim 34, wherein said distal probe
segment has a length in the range of one half inch to one inch.
36. The hand-held instrument of claim 35, wherein said length is
three quarters of an inch.
37. The hand-held instrument of claim 33, said elongate cathode
conductor having a first diameter.
38. The hand-held instrument of claim 37, said elongate anode
conductor having a second diameter larger than said first
diameter.
39. The hand-held instrument of claim 37, wherein said cathode and
anode planar distal end surfaces lie in a plane disposed at an
angle to the elongate cathode conductor axis.
40. The hand-held instrument of claim 39, wherein said plane is
disposed at an angle fifteen degrees from normal to the elongate
cathode conductor axis.
41. A hand-held instrument for monitoring or electrically
stimulating exposed, subcutaneous tissue of a living body,
comprising:
an elongate cannula having an open proximal end, an open distal end
and a lumen there between;
a handle carried on said cannula proximal end;
an elongate, flexible, cathode conductor including a proximal end
and a distal end having a substantially planar distal end surface,
said cathode conductor being malleably deformable and having
sufficient resilience to resist bending when in operable contact
with subcutaneous tissue, said elongate cathode conductor having a
first diameter;
said cathode conductor being disposed within a first flexible
insulating sheath extending distally to and substantially flush
with said distal end of said cathode conductor, said first sheath
leaving said cathode conductor planar distal end surface exposed,
said first sheath including a colored surface;
an elongate, flexible anode conductor including a proximal end and
a distal end having a planar distal end surface, said anode
conductor being malleably deformable and having sufficient
resilience to resist bending when in operable contact with
subcutaneous tissue;
said anode conductor being disposed within a second flexible
insulating sheath extending distally to and substantially flush
with said distal end of said anode conductor, said second sheath
leaving said anode conductor planar distal end surface exposed;
said cathode conductor and said anode conductor being carried
within an elongate non-conductive, flexible jacket having a distal
end, wherein said cathode conductor distal end and said anode
conductor distal end project distally beyond said jacket distal
end; and
said flexible jacket, said cathode conductor and said anode
conductor being carried within said cannula lumen, wherein said
flexible jacket distal end, said cathode conductor distal end and
said anode conductor distal end project distally beyond said
cannula open distal end.
42. The hand-held instrument of claim 41, said first flexible
insulating sheath including a black surface.
43. An instrument for monitory or electrically stimulating exposed,
subcutaneous tissue of a living body, comprising:
an elongate cathode conductor including a proximal end and a distal
end having a distal end surface, said cathode conductor having
sufficient resilience to resist bending when in operable contact
with subcutaneous tissue;
said cathode conductor being disposed within a first insulating
sheath, said first sheath extending substantially to the cathode
conductor distal end and leaving said cathode conductor distal end
surface exposed, said first sheath having a first color;
said cathode conductor distal end surface defining a substantially
planar distally exposed cathode tip;
an elongate anode conductor including a proximal end and a distal
end having a distal end surface, said anode conductor having
sufficient resilience to resist bending when in operable contact
with subcutaneous tissue;
said anode conductor being disposed within a second insulating
sheath, said second sheath extending substantially to the anode
conductor distal end and leaving said anode conductor distal end
surface exposed, said second sheath having a second color visually
distinguishable from said first color; and
said anode conductor distal end surface defining a substantially
planar distally exposed anode tip, wherein said substantially
planar distally exposed anode tip and said substantially planar
distally exposed cathode tip are in a substantially coplanar flush
distal tip configuration.
44. A method for depolarization and stimulation of nerve tissue,
providing repeatable stimulation with greater efficacy,
comprising
a) providing an anode having a first selected wire diameter and a
cathode with a second wire diameter smaller than said anode wire
diameter, wherein the current density proximate the cathode
conductive tip surface is greater than the current density
proximate the anode tip; and
b) stimulating the nerve tissue with said cathode and said anode.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surgical apparatus and more
particularly to a hand held electrical stimulus probe for use as an
intraoperative aid in defining the course of neural structures or
in nerve integrity monitoring. The invention is particularly
applicable for use in monitoring facial electromyogram (EMG)
activity during surgeries in which a facial motor nerve is at risk
due to unintentional manipulation and will be described with
reference thereto, although it will be appreciated that the
invention has broader applications and can be used in other neural
monitoring procedures.
2. Discussion of the Prior Art
Despite advancements in diagnosis, microsurgical techniques, and
neurotological techniques enabling more positive anatomical
identification of facial nerves, loss of facial nerve function
following head and neck surgery such as acoustic neuroma resection
is a significant risk. Nerves are very delicate and even the best
and most experienced surgeons, using the most sophisticated
equipment known, encounter a considerable hazard that a nerve will
be bruised, stretched or even severed during an operation.
Studies have shown that preservation of the facial nerve during
acoustic neuroma resection may be enhanced by the use of
intraoperative electrical stimulation to assist in locating nerves.
Very broadly stated, the locating procedure, also known as nerve
integrity monitoring, involves inserting sensing or recording
electrodes directly within cranial muscles controlled by the nerve
of interest. An electrical stimulation probe is then applied near
the area where the subject nerve is believed to be located. If the
stimulation probe contacts or is reasonably near the nerve, the
stimulation signal applied to the nerve is transmitted through the
nerve to excite the related muscle. Excitement of the muscle causes
an electrical impulse to be generated within the muscle; the
impulse is transferred to the recording electrodes, thereby
providing an indication to the surgeon as to the location of the
nerve.
While intraoperative electrical stimulation has been of benefit in
localization and preservation of facial nerves during various
surgical procedures, the accuracy and reliability of the indication
of stimulation depends upon eliminating sources of false
indications of stimulation. A major source of false indications of
stimulation is the shunting of the electrical stimulus current away
from the intended area and through the body fluids. During acoustic
neuroma surgery the surgical area is invariably bathed in cerebral
spinal fluid (CSF), a clear, colorless body fluid containing
electrolytes and capable of conducting electrical current. The
earliest stimulus probes were crude segments of uninsulated wire or
tapered metal rods touched to the area to be stimulated, often
allowing electrical contact with CSF electrolyte fluid, whereupon
the electrical stimulus current was allowed to spread along the
shunt or parallel paths through the body.
Spreading of the stimulus current reduces the desired electrical
current flowing through the nerve tissue intended for stimulation,
which may result in false negative indications of stimulation and
thus adversely effect the accuracy of the procedure. In the past,
others have attempted to compensate for the problem of current
stimulus spread by simply increasing the intensity level of the
electrical stimulus, whereby the neural response to stimulation
occurs despite the current shunted through undesired paths.
Increased stimulus current levels increase the possibility of
tissue damage, however. In addition, the increased stimulus current
may also spread through undesired paths in inactive tissue,
reaching the active nerve tissue at a level sufficient to produce a
false positive response or indication of stimulation, thus
affecting the accuracy of the procedure, as above.
One of the inventors of the present invention addressed current
shunting problems in the Electrical Stimulus Probe disclosed in
U.S. Pat. No. 4,892,105 (to Richard L. Prass), the entire
disclosure of which is incorporated herein by reference. The probe
of U.S. Pat. No. 4,892,105 has become known as the Prass Flush-Tip
Monopolar Probe and is insulated up to the distal tip to minimize
current shunting through undesired paths. The Prass Flush-Tip
Monopolar Probe is difficult to use when it is desired to provide a
bipolar stimulus, however. Bipolar stimulus is employed whenever it
is desired to provide a current path from anode to cathode through
desired nerve tissue and at controlled depth into the nerve tissue.
In order to provide bipolar stimulus with the Prass Flush-Tip
Monopolar Probe, an anode probe and a cathode probe must be placed
on or near the nerve and held in place during stimulation;
repeatable and consistent placement of the individual monopolar
cathode and anode probes must be maintained in order to avoid
changes in the detection of the stimulus current, possibly leading
to a false response or indication of stimulation, thus affecting
the accuracy of the procedure, as above.
Monopolar probes and Bipolar probes (having integral cathode and
anode tips) are well suited to specific uses. For most applications
of nerve integrity monitoring equipment, the flush tip monopolar
probe (having only a cathode) is selected for initial stimulation
of motor nerves. In operation, the current spreads out in all
directions from the stimulating cathode contact and returns via an
anode contact, usually a needle in the patient's shoulder. Current
spread increases with increasing current levels and is likely to
cause stimulation of any nearby nerve tissue even when the probe
cathode contact is not actually touching the nerve or making
particular good connection to the nerve. Greater specificity or
spatial selectivity can be obtained by operating the probe at
reduced levels of stimulus current. At small levels of stimulus
current, the nerve is stimulated only when the probe is in direct
contact with the neural structure. Accordingly, a balance must be
struck since, as stimulation current is decreased, specificity is
improved but it is more likely that insufficient current will be
provided to stimulate the nerve. At moderate or high levels of
monopolar current stimulation, current may be conducted through
non-neural tissue at levels adequate to stimulate adjacent neural
tissue, causing a false positive response. Moreover, stimulation
current may travel through inactive (non-neural) tissue and
stimulate motor nerve tissue or adjacent neural structures which
may respond simultaneously with the desired neural structure, an
undesirable result when seeking to identify a specific motor nerve.
Monopolar stimulation at moderate to high current levels is
therefore most useful when mapping the course of a selected motor
nerve structure but is not well suited when seeking to stimulate a
single selected motor nerve in an area of the body having many
closely spaced nerve structures, in which case bipolar stimulation
is more likely to be effective.
A bipolar stimulating probe offers increased specificity for
differentiating adjacent neural structures at moderate to high
stimulation current levels. The most important difference between
the bipolar stimulation and monopolar stimulation is that current
flows directly between cathode and anode tips mounted on the distal
end of the bipolar probe instead of going from the monopolar probe
cathode to a distant return anode while spreading in all directions
from the probe tip. The bipolar probe design permits current flow
only from the distal cathode tip to the distal anode tip and
therefore primarily stimulates those neural structures between the
cathode tip and anode tip. Accordingly, monopolar excitation is
preferred when mapping or locating the trajectory of the motor
nerves. Once a motor nerve is located, bipolar excitation is
preferred for use in differentiating among adjacent nerves.
Others have developed bipolar probes providing an exposed anode
conductive tip and an exposed cathode conductive tip, however, the
existing bipolar stimulus probes have not proven entirely
satisfactory.
Most bipolar probe tips are relatively large to accommodate both a
cathode and anode electrode while allowing sufficient
inter-electrode distance to ensure adequate penetration of stimulus
current into the tissue; there is also a problem of handedness,
meaning that a probe may be well suited for use by a left handed or
right handed surgeon or for left or right sided surgery, but not
both.
Since the insulated probe tips have a specific planar orientation,
it may be difficult to accurately place both cathode and anode
probe tips flush on the nerve for precisely targeted stimulation.
Often, it is desired to malleably flex or plastically deform the
probe tips into a more convenient orientation for a given tissue
topography, and the bipolar probes of the prior art fail to
maintain the preselected anode tip to cathode tip spacing after
deformation, thus leading to loss of calibration in the stimulus
current, especially in areas within the body having uneven
topography.
Additionally, when performing surgery under a microscope it is
difficult for the surgeon to determine which tip is the anode and
which is the cathode, and so the surgeon may or may not know the
direction of current flow during stimulation. The bipolar probes of
the prior art also exhibit relatively low efficiency in stimulation
of exposed nerve tissue, as compared to monopolar probes.
There is a need, therefore, for an improved method and apparatus
for providing bipolar stimulation and/or sensing or recording of
electrical activity in the nerve tissue.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to
overcome the above mentioned difficulties by providing an improved
method and apparatus for bipolar stimulation and/or sensing or
recording of electrical activity in the nerve tissue.
Another object of the present invention is efficiently delivering
bipolar stimulus current to nerve tissue or the like.
Another object of the present invention is providing a hand-held
bipolar stimulus probe having cathode and anode conductor tips that
are flexible over the terminal or distal portion and stable in
side-by-side orientation before and after plastic deformation.
Another object of the present invention is providing a hand-held
bipolar stimulus probe having cathode and anode conductor tips with
visually perceptible characteristics permitting the surgeon to
instantly identify cathode and anode tips.
The aforesaid objects are achieved individually and in combination,
and it is not intended that the present invention be construed as
requiring two or more of the objects to be combined unless
expressly required by the claims attached hereto.
In accordance with the present invention, a hand-held bipolar
electrical stimulus probe includes first and second insulated
conductors having unequal diameters and a stable side-by-side
orientation. It was discovered that by varying the diameter of the
cathode and anode conductors, electrical stimulation efficiency was
increased. In a first embodiment, the cathode conductor is a
relatively small diameter (30 AWG) stainless steel wire and the
anode conductor is a larger diameter (24 AWG) stainless steel wire.
By providing a smaller cathode wire diameter, current density
proximate the cathode (-) conductive surface is increased. The
applicants have discovered that the increased cathode (-) current
density provides greater current spread and more efficient and
effective depolarization and stimulation of the nerve tissue,
thereby permitting a repeatable stimulation of the EMG response
with greater efficiency and efficacy than was obtainable using the
bipolar probes of the prior art. The greater current spread is
believed to be attributable to the unequal impedance in the cathode
and anode conductors.
The cathode and anode conductors are provided with visually
perceptible indicia such as color coding for the cathode
insulation, are carried within a flexible jacket and are
individually insulated such that the terminal portions (proximate
the distal ends) are flexible and malleable or plastically
deformable, thereby permitting the surgeon to bend the terminal or
distal probe tips into an orientation for more convenient access to
neural structures. The flexible jacket maintains the cathode tip to
anode tip spacing at a constant or stable distance, both before and
after deformation by the surgeon and during use.
The cathode and anode probe conductors each terminate in a
substantially planar distal tip surface, also known as a planar
conductive surface. The cathode and anode conductors are each
insulated substantially up to the tip to prevent current shunting.
In the exemplary embodiment illustrated and described in greater
detail below, the distance between the cathode and anode planar
conductive surfaces is approximately 0.13 mm, for minimal current
shunting and greater specificity or directivity in applying the
excitation current. This spacing is selected, in part, as a
function of insulator thickness. The bipolar probe of the present
invention thus permits accurate placement for excitation or
monitoring and affords excellent efficiency in electrical
stimulation, in a physically compact and visually unobtrusive
configuration.
The probe preferably includes a molded plastic handle terminated
proximally in first and second electrical connecting pins of
differing diameter. The handle carries a distally projecting
stainless steel rigid tubular cannula having an axial lumen
terminated distally in an open distal end. A flexible plastic
molded jacket is carried within the tubular cannula and projects
distally from the cannula distal end; the flexible jacket extends
distally from the cannula distal end in an unsupported, bendable
length of between 0.5 and 1.0 inches, preferably 0.75 inches.
Alternatively, the flexible jacket portion extends four fifths of
the flexible length that the cathode and anode conductor extend
distally beyond the cannula distal end, leaving one fifth of the
conductor lengths uncovered by the flexible jacket, whereby the
cathode and anode distal tips may be readily viewed.
In the illustrated embodiment, the anode conductor and the cathode
conductor project beyond the flexible jacket distal end by a length
of approximately 1.8 mm and are terminated in slim, flush cathode
and anode tips. By flush is meant that the cathode and anode distal
tips are preferably substantially planar conductive surfaces, each
insulated substantially up to (or flush with) the terminus.
In the preferred embodiment, the cathode and anode distal ends are
disposed in a plane transverse to the probe longitudinal axis.
Alternatively, the planar conductive distal ends can be disposed at
an acute angle with respect to the longitudinal axis of the probe;
in the example disclosed in greater detail below, the cathode
planar tip and anode planar tip are disposed in a plane at an angle
fifteen degrees from normal with respect to the probe axis.
In another alternative embodiment, the bipolar probe includes a
miniature momentary contact single-pole, single-throw switch
integrated into the handle. The switch is wired to select either a
bipolar anode (on the probe tip) or a remote or distant anode
electrode. The probe operates in monopolar mode until the button is
pushed in momentary operation whereupon the probe operates in
bipolar mode. Selective monopolar or bipolar operation is thereby
accomplished using a single stimulus probe. In monopolar mode, only
the cathode (i.e. the smaller conductive probe tip) need be in
contact with nerve tissue for stimulation. Use of the monopolar
mode is desirable when mapping or locating nerve structures or for
enhanced penetration of the stimulus current, such as when locating
a facial nerve in bone, tumor or in inflammatory tissue, whereas
for finer work, such as when discriminating between adjacent nerve
structures, bipolar mode is desirable.
The above and still further objects, features and advantages of the
present invention will become apparent upon consideration of the
following detailed description of a specific embodiment thereof,
particularly when taken in conjunction with the accompanying
drawings, wherein like reference numerals in the various figures
are utilized to designate like components.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view of the bipolar electrical stimulus probe of
the present invention.
FIG. 2 is a side view of the bipolar electrical stimulus probe of
FIG. 1.
FIG. 3 is an enlarged side view of the distal end of the bipolar
electrical stimulus probe of FIG. 1.
FIG. 4 is an enlarged end view of the distal end of the bipolar
electrical stimulus probe of FIGS. 1, 2 and 3.
FIG. 5 is a proximal end view of the bipolar electrical stimulus
probe of FIG. 1.
FIG. 6 is enlarged side view of the distal end of an alternative
embodiment of the bipolar electrical stimulus probe of the present
invention.
FIG. 7 is a side view of the bipolar electrical stimulus probe of
FIG. 1 with a selectively bent terminal portion adjacent the probe
distal end.
FIG. 8 is a side view of the bipolar electrical cord for use with
the bipolar electrical stimulus probe of the present invention.
FIG. 9 is an end view of the bipolar electrical cord socket
connector.
FIG. 10 is an enlarged view of a removable bipolar electrical probe
tip.
FIG. 11 is a schematic diagram in partial cross section of an
alternative embodiment of the probe of the present invention
illustrating the selective operation of the momentary contact
switch for use in monopolar/bipolar probe operation.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring specifically to FIGS. 1,2, 3, 4 and 5 a bipolar
electrical stimulus probe 20 includes an elongate, textured, molded
plastic handle 22 having a proximal end 24 opposing a distal end
26. Handle proximal end 24 carries cathode and anode rounded
cylindrical electrical connecting pins 28, 30 which project
proximally and are spaced apart by a selected spacing. Handle 22
includes a grooved grip area 32 and a transversely projecting
salient tactile locator guide 34 aligned along the longitudinal
axis of the handle, proximate a tapered distal segment terminating
in handle distal end 26 which carries a distally projecting
stainless steel rigid hypo tube or cannula 36 having an axial lumen
terminated distally in an open distal end 38. Cannula 36 is tapered
and preferably has a larger outside diameter proximate the handle
distal end 26, tapering to a smaller outside diameter proximate the
cannula open distal end 38, with a distally projecting length from
handle distal end 26 to cannula distal end 38 encased in clear
plastic, thin-wall, shrinkable tubing.
A flexible plastic molded jacket 40 is carried within the tubular
cannula 36 and projects distally from the cannula distal end 38.
Flexible jacket 40 is preferably made of black ETFE plastic (e.g.,
such as Dupont TEFZEL 200.TM. brand ETFE) and extends distally from
the cannula distal end 38 in an unsupported, bendable length,
terminating in flexible jacket distal end 42. The perimeter of the
cross sectional shape of molded jacket 40 includes a first arcuate
or semicircular segment having a smaller radius and a second
arcuate or semicircular segment having a larger radius, where the
first and second arcuate segments are connected by substantially
straight tangent lines; the smaller arcuate segment is nearer the
smaller cathode tip 50 and the larger arcuate segment is nearer the
larger anode tip 52, as best seen in FIG. 4. The molded on jacket
40 thus provides a relatively uniform coating thickness around the
cathode and anode conductors.
As best seen in FIGS. 2, 3 and 4, bipolar electrical stimulus probe
20 includes a substantially planar, distally exposed cathode tip 50
and a substantially planar, distally exposed anode tip 52 in a
coplanar configuration with cathode tip 50. Cathode tip 50 is the
distal end of an elongate cathode conductor having electrical
continuity with handle cathode pin 28 and anode tip 52 is the
distal end of an elongate anode conductor having electrical
continuity with handle anode pin 30. The cathode conductor is
insulated in enveloping black plastic (e.g., PFA) wire insulation
54 extending substantially up to the distal end of cathode tip 50
and the anode conductor is insulated in enveloping clear plastic
(e.g., PFA) wire insulation 56 extending substantially up to the
distal end of anode tip 52 (as best seen in FIG. 3), thus, the
conductors are provided with visually perceptible indicia (e.g.,
color coding) for cathode and anode identification. When viewed
under a microscope, the surgeon can readily distinguish the black
insulation 54 as identifying the cathode conductor; the applicant
has discovered that black PVA insulation is particularly well
suited to providing a high contrast visual identifying indicia for
cathode (or anode) electrodes used in surgical procedures viewed
under a microscope.
The cathode conductor and the anode conductor project beyond the
flexible jacket distal end 42 by a length of approximately 1.8 mm
and are terminated in slim, flush cathode and anode tips 50, 52. By
flush is meant that the cathode and anode distal tips 50, 52 are
preferably substantially coplanar conductive surfaces, each
insulated substantially up to (or flush with) the tips to prevent
current shunting. Cathode tip 50 and anode tip 52 have different or
unequal diameters and are fixed in a stable side-by-side (or top
and bottom) orientation with spacing therebetween being controlled
by jacket 40. It was discovered that by varying the diameter of the
cathode and anode conductors, or at least the tip diameters,
electrical stimulation efficiency was increased. In the embodiment
of FIGS. 1, 2, 3 and 4, the cathode conductor is a 30 AWG stainless
steel wire with an outside diameter of approximately 0.230 mm and
the anode conductor is a 24 AWG stainless steel wire with an
outside diameter of approximately 0.460 mm. By providing a smaller
cathode wire diameter, current density proximate the cathode (-)
conductive tip surface is increased. The applicants have discovered
that the increased cathode (-) current density provides more
efficient and effective depolarization and stimulation of the nerve
tissue, thereby permitting a repeatable stimulation of the EMG
response with greater efficacy than was obtainable using the
bipolar probes of the prior art. Conversely, applicants observed
that when the cathode and anode leads were switched, thereby making
the anode tip smaller than the cathode tip, less efficient
stimulation was observed at identical stimulus current intensity
settings.
Transversely projecting locator guide 34 serves as a tactile
salient feature aligned with the cathode conductor, thus allowing
the surgeon to use a finger to orient the probe with the cathode
conductor tip 50 in a desired angular direction.
As best seen in FIGS. 3 and 4, the spacing or distance between the
cathode and anode planar conductive surfaces is approximately 0.13
mm; narrow spacing tends to minimize current shunting and provide
greater specificity or directivity in applying the excitation
current. The bipolar probe of the present invention thus permits
accurate placement for excitation or monitoring and affords
excellent efficiency in electrical stimulation, in a physically
compact configuration.
In the preferred embodiment as shown in FIG. 3, the substantially
planar cathode and anode distal tips 50, 52 are disposed in a plane
transverse to the probe longitudinal axis.
In an alternative embodiment illustrated in FIG. 6, a planar
conductive distal cathode tip 50a and planar conductive anode tip
52a are disposed at an acute angle with respect to the longitudinal
axis of the probe. Cathode planar tip 52a and anode planar tip 54a
are disposed in a plane at an angle of approximately fifteen
degrees from normal with respect to the probe axis.
FIG. 7 is a side view of the bipolar electrical stimulus probe of
FIG. 1 with a selectively bent terminal portion proximate the
distal end. The unsupported bendable terminal portion 60 of the
probe 20 comprises the length of flexible jacket (and cathode and
anode conductors) extending distally beyond the rigid cannula tube
distal end 38. The choice of materials determines the ideal
unsupported length for the terminal portion which, in accordance
with the present invention, is malleable or plastically deformable
to retain the shape imparted by the surgeon in bending and adapting
the probe for use in a particular surgery, as best seen in FIG. 7.
During a surgical procedure, the surgeon places the probe distal
cathode tip 50 and anode tip 52 against subcutaneous tissue such as
nerve tissue and applies axial compressive force to maintain
electrical contact between the tissue and the tips 50, 52. The
length, stiffness, resiliency and memory characteristics of the
materials selected for use in probe terminal portion 60 (including
the cathode wire and anode wire) allow the terminal portion to
resiliently retain the selected bent shape and have sufficient
rigidity to provide resilient resistance to bending or buckling
when in operable contact with subcutaneous tissue such as nerve
tissue. The applicants determined experimentally that it was
especially difficult to keep the spacing of differently sized
cathode and anode conductors constant or stable during and after
deformation by the surgeon and during use. The applicant's solution
to the problem of stability is providing the thin, flexible jacket
or insulator sleeve 40 extending distally from the inflexible
cannula distal end 38 for a length of at least four fifths of the
length of the terminal portion 60; thereby leaving adequate
flexibility and visibility of the tissue contacting distal tips 50,
52.
FIG. 8 is a side view of the bipolar electrical cord 70 for use
with the bipolar electrical stimulus probe 22; electrical cord 70
has a distal dual socket connector 72 providing continuity to first
and second proximal two mm diameter connector pins 74, 76, via a
selected length (e.g., approximately two meters) of flexible, 24
AWG, two-conductor, unshielded duplex wire 78. When in use,
connectors 74 and 76 are plugged into corresponding sockets in a
patient interface module of a nerve integrity monitoring instrument
(such as the Xomed.RTM. NIM-2.RTM. XL Nerve Integrity Monitor).
FIG. 9 is an end view of the bipolar electrical cord socket
connector 72 and shows the cathode socket 80 having a diameter of
0.062 inches and the anode socket 82 having a diameter of 0.072
inches; the cathode and anode sockets 80, 82 are sized to receive
and provide an electrical connection with the probe connector pins
28, 30.
FIG. 10 is an enlarged view of an embodiment of the bipolar probe
including a removable bipolar electrical probe portion 92 including
a cathode pin connection 94 and an anode pin connection 96 shown
withdrawn from corresponding cathode and anode socket connections
in the distal end 98 of the handle.
In the preferred embodiment of the probe of the present invention,
cathode and anode rounded cylindrical electrical connecting pins
28, 30 are spaced approximately 0.220 inches apart. Cathode pin 28
has a diameter of 0.062 inches and a length of 0.30 inches, anode
pin 30 has a diameter of 0.072 inches and a length of 0.30 inches.
Handle 22 has an axial length of 3.88 inches and transversely
projecting salient tactile locator guide 34 is aligned along the
longitudinal axis of the handle, proximate the tapered distal
segment terminating in handle distal end 26 which carries a
distally projecting stainless steel rigid hypo tube or cannula 36
having an axial lumen terminated distally in an open distal end 38.
Cannula 36 tapers with an outside diameter of 0.13 inches proximate
the handle distal end 26, has an outside diameter of 0.080 inches
proximate the cannula open distal end 38 and has a distally
projecting length from handle distal end 26 to cannula distal end
38 of approximately 2.3 inches.
Flexible jacket 40 is preferably made of black ETFE plastic (e.g.,
such as Dupont TEFZEL 200.TM. brand ETFE) and extends distally from
the cannula distal end 38 in an unsupported, bendable length of
between 0.5 and 1.0 inches, preferably 0.75 inches.
FIG. 11 is a schematic diagram in partial cross section of an
alternative embodiment of the probe of the present invention
including an anode selective bipolar probe 100 incorporating a
miniature momentary contact single throw, single pole, three
terminal switch 102 for selective use enabling both monopolar and
bipolar modes of probe operation. The anode selective bipolar probe
100 preferably includes a malleable terminal portion having a
cathode tip and anode tip with the same geometry and dimensions as
specified for the bipolar probe of FIGS. 1-4.
The momentary contact switch 102 is integrated into the probe
handle 104 and is actuated by a button 105 preferably located
adjacent the handle locator guide 106. The three terminals of
switch 102 are wired with a first terminal 110 connected to the
probe anode conductor 112. The second switch terminal 114 is
connected to a remote anode 116 which is positioned against the
patient's body, as is customarily done in monopolar excitation. The
third switch terminal 118 is connected to anode connector pin 74a.
Cathode connector pin 76a is hard wired to the cathode conductor
120 and is not affected by operation of switch 102. Connector pins
74a and 76a are adapted for connection to corresponding sockets in
the patient interface module of a nerve integrity monitoring
instrument, as above.
Momentary contact switch 102 is wired to select either the probe
tip anode, for bipolar probe operation, or remote anode electrode
116, for monopolar probe operation. Anode selective bipolar probe
100 operates in monopolar mode until button 105 is pushed in
momentary operation. Selective monopolar or bipolar operation is
thereby accomplished using a single anode selective bipolar probe
100. In monopolar mode, only the cathode (i.e. the smaller
conductive probe tip) need be in contact with nerve tissue for
stimulation.
When used for head and neck monitoring with nerve integrity
monitoring equipment, the monopolar mode is selected (by not
depressing button 105) and the stimulus current spreads out in all
directions from the stimulating cathode contact and returns via the
anode contact 116 (e.g., a needle electrode in the patient's
shoulder such as is disclosed in U.S. Pat. No. 5,161,533, to Prass,
et al, the entire disclosure of which is incorporated herein by
reference). Current spread increases with increasing current levels
and will likely cause stimulation of nerve tissue even when the
probe cathode tip is not actually touching the nerve or making
particular good connection to the nerve. At moderate or high levels
of monopolar current stimulation, adjacent neural structures may
respond simultaneously with the desired neural structure. Use of
the monopolar mode of stimulation at moderate to high current
levels is therefore most useful when mapping the course of a
selected motor nerve structure but is not well suited when seeking
to stimulate a single selected motor nerve in an area of the body
having many closely spaced motor nerve structures, in which case
use of the bipolar stimulation mode is more likely to be
effective.
The bipolar mode of operation offers increased specificity for
differentiating adjacent neural structures at moderate to high
stimulation current levels, and is accomplished by depressing
button 105 on the handle of anode selective bipolar probe 100. The
most important difference between the bipolar mode of stimulation
and the monopolar mode of stimulation is that current flows
directly between the two tips of probe 100 instead of going from
the probe cathode to the remote return anode 116 and spreading at
all directions. The bipolar mode of excitation permits current flow
only from the probe distal cathode tip to the probe distal anode
tip and therefore stimulates only those neural structures between
the cathode tip and anode tip. Accordingly, monopolar excitation is
preferred when mapping of locating the trajectory of the motor
nerves, and once a motor nerve is located, bipolar excitation is
preferred for use in differentiating among adjacent nerves.
In an alternative embodiment adapted for direct recording of
compound nerve action potentials from sensory nerves, motor nerves
or brain tissue; greatest spacial selectivity may be obtained with
a bipolar probe structure having anode and cathode electrodes of
equal diameter.
Having described preferred embodiments of a new and improved
method, it is believed that other modifications, variations and
changes will be suggested to those skilled in the art in view of
the teachings set forth herein. It is therefore to be understood
that all such variations, modifications and changes are believed to
fall within the scope of the present invention as defined by the
appended claims.
* * * * *